(1) In establishing the seniority of teachers the Commission shall take account of the following in that order—
(a) years of service in current grade and date of appointment to that grade;
(b) years of service in previous grades and dates of appointment to each previous grade.
(2) The seniority of a teacher who voluntarily resigns from the teaching service and who is subsequently re-appointed shall be determined by the date of his or her last appointment.